Electroplating process



UNITED STATES PATENT OFFICE ELECTBOPLATIN G PROCESS Floyd F. Opll'nger, Niagara Falls, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application July 30, 1984, Serial No. 737,569

a Claims. (Cl. 204-11) This invention relates to electroplating metals 'ings may be Obtained by Plating under certain and more particularly to a method of producing carefully controlled conditions or by bufling and bright, smooth coatings of electroplated tin, cadpolishing, such plates are very easily tarnished mium, zinc and their alloys. and finger-stained. While it is possible to obtain Electroplated coatings of tin, cadmium, zinc t surface 011 a Plated article y buffing 5 and alloys of these metals when taken directly and Polishing Without undue removal 0f Coated from the electroplating bath ordinarily have a metal, s difficult and ly impractical t0 dull finish and are very susceptible to fingerobtain a uniform bright coat on articles of irregstaining and to tarnishing. For most purposes ular shape because of the difliculty of buffing 10 a bright, non-tarnishing finish is desirable. Orand polishing deep d p ssions a d e ular lo dinarily such electroplated coating is brightened Surfacesby buffing and polishing; this, however, does not An object of the present invention is to prosubstantially reduce the tendency to tarnish. ime S ooth. brilglht, el c p t d c sh 0f Coatings of these metals, having a brig fusible metals suc as tin, zinc, cadmium or t eir pearance, also may b bt by dipping t alloys withciautt the use of mechanical polishing 1;, metal articles to be coated in a molten bath of Operations, a h add 1 age ts, etc. A furl er the coating metal. In order to obtain a smooth, Object is to Produce Such coatings 011 iron uniform and bri ht surface by t method it is steel. A still further object is to produce electrousually necessary to further treat the coated Plated coatings of cadmium, Zinc their articles, e, g, by rolling wiping or other treatalloys on iron or steel which coatings have supement t re excess metal and smooth the rior adherence to the base metal and increased c0atings while t t dipping method f corrosion resistance. Other objects will appear coating'metals is capable of producing a bright, from e) o ovingtdescription of my invention. smooth surface, it has a number of dlsadvan- The a (We 0 5 are accomplished by elect In t t place, when iron or steel troplating metallic articles with metals fusible articles are thus coated, careful and-tedious prepbelow the meltmg point of the base and Subject amtion of t metal urfa e to remove oxide ing the electroplated articles to a heat treatment and other impurities are necessary before the unger i gg nin'oxidizting conditions at metal'is hot dipped in order to obtain a smooth empem a he mel ing Point of the adherent coat. These preliminary steps include coated metal- Preferab1y the coated metal is U pickling operations and annealing. Furthermore, heated to a temperature'shghtly above its melt Datals which are coated by hot dipping have a ing point for a short time, for example 10 to 30 relatively thin layer of coated metal because of Secondsmveniwn also compltlses a novel the fact that the molten metal has a relatively method .Pwkling 9 or steel artlcles prior to low viscosity and when the article is removed ezflctmplatmg as Peremafter described which 35 from the hot dipping bath excess metal drains S ts in Supenor electroplated coating- By off, leaving a relatively thin coating on the arti- 2 i method? I have. tamed cle. The thickness of the coating is further ret g? fi of i tin duced when a wiping or rolling operation is used. g ggg g g g' fi arac nzed by e 10w 40 Hot dipping methods are also in general not I am aware that it has been proposed readily applicable to odd shaped or deeply reprove electroplated coatings by the application cessed articles. Frequently great difiiculty is ente d i h t di 1 th d5 b th of heat. However, so far as I am aware, heretocoun re n 0 pp ng o ecause e fore it has not been proposed to electroplate metmetal to be coated tends to dlssolve in the coatals and subsequently heat the electroplated Sub ingmetal so that the coatmg metal bath becomes fa to a temperature above the melting point rapldly contamma'ted- I under substantially non-oxidizing conditions to In many cases an electroplating process is obtain the smooth, bright, oxide-free coating preferable for coating metals because less prepam characterizes my invention,

5 ration of the me al pri r o pl in is required I have discovered that when heavy base metals and a thicker coating may be obtained. Howsuch as iron, copper or brass are electroplated v r, heretofore n t ly satisfactory process with metals having a melting point lower than has been proposed for producing bright electrothe base metal, such as zinc, cadmium, tin or plated coatings of soft metals such as tin, cadtheir alloys and the electroplated article is sub- 5 mium or zinc. While bright electroplated coatsequently heated under substantially non-oxidizing conditions to a temperature above the melting point of the base metal, a bright smooth plate is obtained which has a low degree of porosity. Such plate is similar in appearance to that obtained by the best methods of hot dipping or hot galvanizing and may be made of a much greater thickness. I have further discovered that such base metals, especially iron or steel may be plated with improved results if prior to plating the metal is treated with a strong acid to such extent that a distinct and uniform etching corrosion of the surface is obtained, followed by an alkaline treatment. With such pretreatment, the adherence of the plated coating is improved and the porosity of the plate is materially decreased. I have further found that when such pretreated metal is plated with tin, cadmium, zinc or their alloys and the plated coating is heat treated to a temperature above its melting point as described above, the resulting plate is smoother and more uniform than is the case when the base metal is not so etched and alkali treated prior to plating.

The above-mentioned etching prior to electroplating is especially useful in obtaining adherent, non-porous coatings of tin or the rust-resisting metals on cold-rolled sheet steel. Cold-rolled steel ordinarily has a bright, smooth surface. When bright, cold-rolled steel is electroplated for example with tin, the plated metal has relatively poor adherence and corrosion resistance. However, if the steel is previously treated with acid and alkali as described above, the electroplated tin has good adherence to the base and the corrosion resistance is markedly improved, without increasing the depth of the electroplated layer. These improved results are especially marked when the electroplated coating is in the neigh borhood' of 0.0001 inch thick or heavier.

One method of practicing my invention will be illustrated by reference to the production of a smooth, bright coating of tin or tin alloy on steel. The surface of the steel after suitably cleaning to remove grease and dirt, is first treated by immersion in a strong acid solution preferably at an elevated temperature, e. g. -200 F., until the surface of the metal is distinctly etched so that a uniformly etched or corroded surface is plainly visible to the unaided eye. The steel is then treated with an alkaline solution, preferably by making the steel the anode in an alkaline cyanide bath and applying a moderate electric current for a minute or two. The treated steel is electroplated with tin or tin alloy by any suitable method to produce a tin layer of the described thickness. I prefer to elec-. troplate from an alkaline solution, especially by the methods described in U. S. Patents 1,841,978 and 1,919,000. These or similar methods may be used for plating tin or tin alloys on the treated iron or steel.

After the desired thickness of tin or tin alloy has been placed on the steel, the article is heated under substantially non-oxidizing conditions to a temperature above ti melting point of the tin or tin alloy coating. If the coating is of substantially pure tin (which melts at about 450 F.), heating may be to a temperature of about 460 to 500 F. for a period of ten to thirty seconds.

Various known means of heating under nonoxidizing conditions may be utilized in order to carry out my invention. -For some work and with relatively low melting coatings I have found a bath of molten tallow to be suitable; for other pieces and especially for -continuous operation, or high melting coatings, a mufile furnace having an atmosphere of a non-oxidizing gas such as hydrogen or nitrogen or a furnace having small openings for the entrance of the articles to be heated and provided with means for continuously maintaining a substantially non-oxidizing atmosphere of such non-oxidizing gas may be utilized.

My invention will be further illustrated by the following examples.

Example 1 Samples of cold rolled sheet steel (automobile body stock) were first freed from grease and dirt by cleaning in an alkaline electrolytic cleaner containing:

Oz./gal. Sodium cyanide 2 Caustic soda 2 Trisodium phosphate 4 Temperature of solution-160-180 F. making it cathode and anode alternately, until entirely free from grease and dirt.

The samples then were pickled in a sulfuric acid solution containing 15% by volume of sulfuric acid (66 B.) at a temperature of 70-90 C. for a period of 4 minutes. Aside from the removal of oxide or scale, the pickling was continued until the rolled surface layer was removed and a distinct etching was visible to the naked eye.

The samples then were water rinsed and further treated electrolytically in a sodium cyanide solution containing 4 oz./gal. of NaCN at F., by making it the anode at 100 A/SF for 1 minute. After this treatment the samples again were water rinsed and then tin plated in an alkaline tin solution containing:

Oz./gal. Sodium stannate 12 Caustic soda 1 Sodium acetate 2 Hydrogen peroxide (100 vol.)

Data

Anode Straits tin Ratio of anode to cathode area 3 to 1 Cathode current density 10 to 50 A/SF E. M. F 4.0 to 6.0 volts Temperature of solution 60 to 80 C. Time of plating 3 to 20 minutes Thicknesses of 0.00005" to 0.0004" of tin were produced.

Following the plating, the samples were rinsed in cold and hot water and. dried. The dry samples were immersed for 10 to 20 seconds in a molten tallow bath at a temperature of 240- 260 C. This treatment produced bright, smooth coatings of tin similar in appearance and hardness to that produced by hot tinning processes.

Example 2 Samples of cold rolled sheet steel were cleaned and then treated with acid and cyanide solution as described in Example 1. The treated samples were placed in an acid tin solution made up as follows:

Oz./gal. Sodium stannate 6 Sulfuric acid 16 Cresylic acid 1 Glue /2 mentioned in the Following the plating, the samples were heated in a tallow bath as described in Example 1, with substantially identical results.

Example 3 Cold rolled sheet steel samples were .treated, plated and heat treated as described in EX-a ample 1, except that the following acid solution was used to etch the steel:

The procedure of Example 3 was repeated except that the following acid solution was used to etch the steel:

HNO3 (cone) Sulfuric (cone) 1 part by volume 1 part by volume Water 4 parts by volume Temperature normal room temperature Time of immersion 30 seconds to 2 minutes Example 5 Samples of hot rolled steel were first freed from grease and dirt by cleaning at 160 to 180 F. in an alkaline electrolytic cleaner containing:

The work was made cathode and anode alternately until entirely free from grease and dirt.

The samples were then pickled in a sulfuric acid solution containing 15% by volume of sulfuric acid (66 B.) at a temperature of to for a period of l to 3 minutes so that all fire scale was removed and etching was plainly visible to the naked eye.

The samples were then water rinsed and further treated in a warm sodium cyanide solution containing 8 oz./gal. of NaCN at 140 to 160 F., by making them anodes at a current density of 50 to A/SF for 1 minute. After this treatment, the samples were plated and heat treated as in-Example l, with substantially the same results.

Example 6 In place of the anodic cyanide treatment foregoing examples, sheet steel samples, after being etched in various acid solutions, were subjected to anodlc treatment at about 70 C. in a' solution containing:

Oz./gal. Sodium hydroxide 2 Sodium carbonate 4 The anodic treatment was continued for 1 to 2 minutes at a current density of 50 to 100 A/SF. The samples were then plated and heat treated as in Example 1, with the same results.

mersed for 3 minutes at as in Example 1.

Data Example 7 Anodes Straits tin Samples of sheet brass and copper were first 3 3 anodet 2 t 'z'jf'a-i g cleaned by making them cathodes for 3 to 5 o e curren ens y I minutes at to F. in an alkaline solution E. M. F 2.0 to 4.0 volts containing. Temperature of Oz /ga1 solution normal room temperature Sodium cyanide Time of plating 3.0 to 20 minutes medium f 2 Thicknessof caustic sod... 4

The cathode current density was 50 to 100 A/SF. After water rinsing, the samples were pickled at 140 to 160 F. for 2 to 4 minutes in a solution containing:

. Part by volume Water 1 Murlatic acid 1 rinsed and im- F. in a solution containing 4 oz./gal. of sodium cyanide; The treated samples then were plated and heat treated as in Example 1.

The samples then were water Example 8 Samples of sheet brass and copper, cleaned as described in Example 7, were treated in the following solution:

Cc./L Sulfuric acid 66 B 530 Nitric acid,42 B 160 Hydrochloric acid 24 B 0.75 Water 320 Example 9 Pieces of cold-rolled sheet steel were first cleaned to remove grease and dirt as in Example 1. They were then plated without previous acid etching or alkali treatment in a tin solution similar to that of Example 1 to which had been added about oz./gal. of cadmium oxide dissolved in sodium cyanide solution. The plated coatings had a thickness of 0.0001" to 0.0004. After plating, the coatings were treated in molten tallow at about 260 0. Evidence of the presence of cadmium in the coating was presented by the fact that the coatings melted below 231 C., the melting point of tin.

Bright deposits of cadmium-tin alloys were produced in all cases by the heat treatment. However, the coatings were lumpy, due to the omission of the acid etch and alkali treatment.

Example 10 Pieces of cold-rolled sheet steel were degreased They were then etched for 3 to 5-minutes in a 15% solution of H2304 at a temperature of 70 to 90 C. After the acid treatment, the pieces were water rinsed and then treated anodically in a sodium cyanide solution (6 oz./ga1.) at 140 to 160 F., using a current density of 50 to 100 A/SF for 1 minute. After water rinsing the pieces were then plated in the tin-cadmium bath described in Example 9. Upon heat treatment in tallow as described in Example 9; adherent,. bright coatings of cadmium-tin alloy were obtained. These coatings were smooth and entirely free from lumps.

Example 11 Pieces of cold-rolled sheet steel were degreased as described in Example 1. They were then acid etched as described in Example 13. Following this the same anodic treatment as in Example 13 was used.

The pieces were then plated with tin as described in Example 1 to a thickness of 0.0001" to 0.0003' Following this, they were plated with cadmium from a solution containing:

Oz./gal. Cadmiumoxide 4 Sodium cyanide 10 Sodium hydroxide 1 Data Anodes Cadmium Cathode current density 20 to 30 H/SF Temperature Normal room temperature Coatings of cadmium 0.0001" to 0.0003"-thick were applied to the previously plated tin coating so that the total thickness of the coatings varied between 0.0002" and 0.00006". These coatings were then heat treated in tallow at 200 to 300 C.

Adherent bright coatings of tin-cadmium alloy, free from lumps or runs were obtained.

Example 12 Pieces of cold-rolled steel were prepared and plated with 0.0001 to 0.0003" of tin as described in Example 11. Following the tin plating, the samples were zinc plated in a solution containing:

Oz./gal Zinc cyanide 5 Sodium cyanide 3 Sodium hydroxide 4 Data Anodes Zinc Cathode current density 20 to 30 H/SF Temperature 40 to 50 C.

The tin plated pieces were zinc plated with 0.0001 to 0.0002" of zinc.

Upon being heat treated in tallow at 200 C. C., adherent, bright coatings of tin-zinc alloy were obtained. The coatings were free from lumps and other imperfections.

Example 13 Pieces of hot-rolled sheet steel were degreased in an alkaline electrolytic cleaner and then acidpickled to remove rust and fire scale until a clean surface was .obtained. Following the pickling, the pieces were immersed in a NaCN solution (6 0z./gal.) and cleaned anodically for 1 minute using 50 to 100 A/SF.

The samples were then plated as described in Example 11 with 0.0002" to 0.0003" of cadmium. The cadmium plated pieces were then plated in a zinc solution as described in Example 12 with 0.0001" to 0.0002" of zinc. The double plated pieces were then treated in tallow at 260 to 300 C. Adherent bright coatings of cadmiumzinc alloy were obtained free from lumps and other imperfections.

As mentioned above, in practicing my process I prefer to treat the metal to be plated, especially iron or steel, first with a strong acid under such conditions and for such length of time that a decided visible etching corrosion of the metal is obtained and then to treat the metal with an alkaline solution. This method of pre-treatment of iron and steel markedly decreases the porosity of the electroplated coating and hence increases the corrosion resistance of the plate. This eifect is not limited to the low-melting metals such as tin, zinc or cadmium; for example, I have found that the corrosion resistance of nickel plate is improved by this pre-treatment. Furthermore, when it is desired to subsequently heat treat the electroplated coating by the herein described method, this preliminary etching and alkaline treatment assists in producing a smooth coating of the final product, especially when the electroplated coating is relatively thick.

coatings are heat treatto produce the required etching effect. I prefer to use a strong acid solution, that is about 2 normal to 6 normal strength and carry out the etching operation at an elevated temperature, for example 100 to 200 F.

The time required to etch the metal with acid and/or the type of acid or acid mixture required may vary, depending on the nature of the metal. The kind of acid required to corrode a given metal will be apparent to one having a knowledge of chemistry. We have found that different samples of cold-rolled steel require diiferent acid treatments in order toobtain a satisfactory etching. These differences apparently depend partly upon the constitution of the steel and partly on the heat treatment and mechanical working to which it has been subjected. The best method of etching a given sample may easily be determined by simple trials with various acids or mixtures atyarious temperatures. When it is desired to produce a mirror-like plate by the herein described heat treatment after plating, the steel should not be etched too drastically. However, even when the steel is drastically etched, the heat treatment subsequent to plating produces'a distinct brightening and a smooth, dense plate.

' Various alkaline aqueous solutions are suitable for the alkali treatment following the above de-' scribed etching operation. I have found that in general the best results: are obtained when the alkaline bath contains substantial amounts of an alkali metal cyanide; if desired, a solution of alkali metal cyanide alone may be used or sodium hydroxide or other alkaline materials maybe added. I prefer to use a cyanide solution containing 2 to 10 oz. per gal. of sodium cyanide as an electrolytic cleaning bath, wherein the work to be treated is preferably made the anode and using an anode current density of up to around 100 amps. per sq. ft. The best results generally are secured by keeping the bath hot, e. g. at a temperature of 140 to 160 F. and using an anode current density of 50 to 80 amps. per sq. ft. Various other alkaline solutions may be used for such electrolytic treatment. The time of treatment may be varied widely; ordinarily 3 to 3 minutes is sufficient when the electrolytic method is used. If the alkaline bath is used without the aid of the electric current, a somewhat longer time of treatment usually will be required, eg up to around 30 minutes.

As mentioned above, my process is suitable for coating metals with alloys of tin, cadmium, or zinc as well as the pure metals. Various other relatively fusible metals or alloys, e. g. lead or lead alloys, may be utilized in my invention. The alloy coatings may be obtained by electroplating the alloy from electrolytes containing salts of alloy constituents. An alloy coating also may be obtained in accordance with my invention by plating out the metals to be alloyedv in two or more separate layers and then subjecting the electroplated article to the-above described heat treatment under non-oxidizing conditions. For example, an article may be plated with a layer of tin and then with a layer of cadmium or the tin may be deposited on the cadmium plate and the article thus double plated subjected to a heat treatment under non-oxidizing conditions at a temperature slightly above the melting point of the resulting timcadmium alloy. Likewise, other metals may be alloyed by this method with tin,

cadmium or Line; for example, a tin-copper alloy may be made by first plating the article with tin and then with a light copper plate and subjecting the electroplated articles to the above described heat treatment. In this case, in order to produce a tin-copper alloy at the surface, it is essential that the copper be plated over the tin. If the tin is plated over the copper, I have found that complete alloying of the copper and tin cannot be obtained without the use of a heat treatment at a temperature far above the melting point of tin. Such high temperatures are in many cases impracticable to use because of the effect they may have on the base metal. Hence, in general when it is desired to produce an alloy cf tin. cadmium, or zinc with another metal such as copper or silver, which has a markedly higher melting point, by means of the multiple plating method, I prefer to plate the higher melting metal as the exterior coat prior to heat treating.

Various known methods of heat treating under non-oxidizing atmosphere may be used; I prefer to use a bath of hot tallow, or other high boiling liquids, fused salts or the like; or a furnace which is continuously provided with an atmosphere of oxygen-free hydrogen. Obviously these preferred methods may be modified by using other suitable non-oxidizing liquid or gaseous media.

An advantage of my herein described invention is that it results in a smooth, bright plated coating which has superior corrosion resistant properties. The coating has the density, hardness, low degree of porosity and bright appearance' which are characteristic of hot dipped coatings and furthermore, my coatings may be made of greater thickness than by hot dipping methods. Also, the step of heat treating subsequent to plating in accordance with my invention simplifies the electroplating operation, since it eliminates the need of originally obtaining plates having the best color and general appearance. The heat treatment brightens the dullest plates substantially as well as the brighter ones. Hence it is necessary only to regulate the electroplating bath to obtain plating suitable for heat treatment.

Furthermore, my improved method of treating metal prior to plating is advantageous not only in producing superior coatings byheat treating subsequent to plating, but also improves the electroplate itself as regards adherence and corrosion resistance. Hence, improved results may be obtained by this method without heat treating subsequent to plating. V

I claim:

1. A process for producing an alloy coating on a ferrous metal comprising treating said ferrous metal with an acid until the surface thereof is visibly etched, then treating said metal with an alkaline solution for a period of at least 3 minutes, electroplating the treated metal with a plurality of different metals applied in successive layers and finally heating the electroplated surface under substantially non-oxidizing conditions to a temperature sufliciently high to cause the said electroplated layers to substantially completely fuse together to form an alloy coating.

2. A process for producing an alloy coating on a ferrous metal comprising treating said fer- -rous metal with an acid until the surface thereof is visibly etched, then subjecting said metal to anodic treatment in an alkaline solution, electroplating the treated metal with a plurality of different metals applied in successive layers and finally; heating the electroplated surface under substantially non-oxidizing conditionsto a temperature sufficiently high to cause the said electroplated layers to substantially completely fuse V together to form an alloy coating.

3. A process for producing an alloy coating on a ferrous metal comprising treating said ferrous metal with an acid until the surface thereof is visibly etched, then subjecting said metal to anodic treatment in an alkaline solution, electroplating the treated metal first with a layer of tin and then with a relatively thin layer of cop- FLOYD F. OPLINGER. 

